Dinosaurs behaving badly: Did velociraptors hunt in packs?

Reconstructing dinosaur behaviour – such as the hunting strategy of raptors – is fraught with difficulty, writes
Brian Switek

Palaeontologist Dominic Richards poses with the dinosaurs ahead of the premier of Walking with Dinosaurs at the 02 Arena on 5 August 2009 in London. The show, based on an award winning TV series, features dinosaurs including Stegosaurus, raptors and a T Rex. Photograph: Dan Kitwood/Getty Images

When I first saw Jurassic Park in the summer of 1993, I was stunned. The visions of sluggish, stupid reptiles in my elementary school's library books were crushed in the jaws of hot-blooded, supercharged dinosaurs, including the fearsome Velociraptor. Small, agile and smart, these hunters relied on cooperation to stalk their human prey. "Clever girl", indeed.

Jurassic Park's dinosaurs were not entirely accurate, of course. Tyrannosaurus probably could not have kept up with a speeding jeep and Dilophosaurus was not a frilled venom-spitter, but the pack-hunting behaviour of Velociraptor was not a Hollywood invention.

In 1969, palaeontologist John Ostrom described a strange dinosaur from the 110m-year-old rock of Montana. Named Deinonychus, this roughly human-sized predator had grasping hands, a hyperextendable sickle claw on each foot, and a stiff tail that acted as a dynamic counterbalance. Presented as the antithesis of the reptilian dinosaur archetype, Ostrom concluded that Deinonychus "must have been a fleet-footed, highly predaceous, extremely agile and very active animal, sensitive to many stimuli and quick in its responses."

Ostrom also thought that Deinonychus was a pack hunter. At least three Deinonychus were found alongside the herbivorous dinosaur Tenontosaurus at a quarry excavated by Ostrom and his colleagues, and numerous Deinonychus teeth were discovered among the remains of the same prey at fourteen other sites. (Since dinosaurs replaced teeth throughout their lives, predators could occasionally afford to lose a tooth or two while feeding.) Where Tenontosaurus bones were found, traces of Deinonychus frequently turned up.

Tackling a Tenontosaurus wasn't easy. Despite lacking armour or spikes, an 8-metre adult Tenontosaurus would have been hefty enough to break the bones of an attacker. Killing such a large animal would have required cooperation, and this conjecture – along with the common association between the two species – fuelled the idea that packs of Deinonychus often pounced upon poor Tenontosaurus. The gory conflicts were immortalised in museum displays and palaeo-art ever after and, given a name change, Deinonychus used the same tactics in Jurassic Park.

But not everyone has agreed that Deinonychus hunted in packs. In 2007 palaeontologists Brian Roach and Daniel Brinkman argued that the Tenontosaurus kill sites Ostrom cited represented bloody scrambles where individual Deinonychus scrapped over feeding rights. In Ostrom's view, the three partial Deinonychus skeletons that inspired his hypothesis were individuals that were killed while bringing down the Tenontosaurus, but Roach and Brinkman argued that the three were slaughtered by other Deinonychus during competition for the carcass. The dinosaurs were more like komodo dragons than wolves.

Extrapolating behaviour from scattered skeletons is tricky. As Ostrom himself noted, the associations between the Deinonychus and Tenontosaurus bones can be interpreted in different ways. Roach and Brinkman's view is one alternative, but is it correct? The idea that Deinonychus hunted like komodo dragons rests on assumptions and circumstantial evidence, just as Ostrom's pack-hunting hypothesis did, and, at the moment, the stories of the bones remain ambiguous.

Nevertheless, exceptional trackways have shown that raptors were social at least some of the time. Deinonychus and its kin – called dromaeosaurs – only stood on two toes, with their wicked sickle claws held off the ground. Their distinctive tracks are rare, but in 2008 Rihui Li and co-authors described footprints of a Deinonychus-like dinosaur from the Early Cretaceous of Shandong, China. In a single bedding plane, the scientists found the footprints of at least six individuals that walked in close proximity to each other and in the same direction. Footprints are fossilised behaviour, and in this unique case the trackways recorded the movements of a raptor pack.

The tracks from China are not the only evidence for gregarious raptors. Just last month palaeontologists Alexander Mudroch and colleagues reported over 120 impressions left by Deinonychus-sized dromaeosaurs in the Mid-Jurassic strata of Niger. There were five distinct trackways made at different times, but one pair of trackways was left by two dinosaurs moving together in the same direction. One of the trackways even shows how one dinosaur abruptly changed speed to avoid running into its companion.

We don't know for sure how raptors hunted. The footprints from China and Niger show that these dinosaurs were social at least some of the time – they didn't bite each other's heads off as soon as they saw each other – but we can't assume that they would have cooperated just like wolves or lions. We need more evidence, and, rather than try to shoehorn them into this or that behavioural repertoire, perhaps we should do as Robert Bakker once suggested and "Let dinosaurs be dinosaurs," as we search for clues about their lives.